The present invention relates to the field of seismic sources for marine geophysical operations. More particularly, the invention relates to an improved seismic source using piezoelectric elements within a slotted housing to generate seismic source energy.
Marine seismic vessels tow vibrators, air guns, explosives, and other acoustic projector techniques to generate seismic source energy in marine geophysical operations. The seismic source energy is represented by a pressure pulse in the water. The pressure pulse generated travels downwardly through the water and underlying geologic structures and is reflected from interfaces between the geologic structures. The reflected signal impulses return to the water column and are detected with sensors towed behind the seismic vessel or laid on the water bottom.
Seismic operations typically use high powered acoustic signals near 190 dB/Hz re micro Pascal in a low frequency band between 5-120 Hz. High power signals penetrate deep within subsurface geologic structures, and low frequency signals experience less attenuation than higher frequency signals.
High power, low frequency source signals for marine geophysical operations are typically generated with air guns or other acoustic sources. U.S. Pat. No. 3,896,889 to Bouyoucos (1975) disclosed a mass oscillation system for generating acoustic source energy in water. Other devices generate an acoustic signal by transmitting high velocity water jets into the underwater water environment. U.S. Pat. No. 4,131,178 to Bouyoucos (1978) and U.S. Pat. No. 4,153,135 to Bouyoucos (1979) disclosed a moveable piston for generating high velocity water jets. U.S. Pat. No. 4,234,052 to Chelminski (1980) disclosed another liquid jet acoustic source system. Improvements to liquid jet acoustic source systems were disclosed in U.S. Pat. No. 4,695,987 to Buoyoucos (1987) and U.S. Pat. No. 4,753,316 to Buoyoucos et al. (1988).
Another type of seismic source system releases compressed air through various mechanical firing systems into the water. U.S. Pat. No. 4,180,139 to Walker (1979) disclosed one type of air gun, and U.S. Pat. No. 4,285,415 to Paitson (1981) disclosed a mechanism for controlling the discharge of compressed air. U.S. Pat. No. 5,228,010 to Harrison (1993) disclosed a shuttle air gun for generating acoustic source energy.
Underwater acoustic vibrators have been used in submarine operations to detect and to locate the position of vessels and underwater objects, however such vibrators do not operate at the power levels and frequency necessary in seismic operations. An example of a high frequency, low power system is described in U.S. Pat. No. 3,875,552 to Hogman et al. (1975), wherein sonar signals were transmitted from a mobile, underwater target, and in U.S. Pat. No. 5,600,087 to Chace (1997), which described a programmable underwater vehicle. These acoustic vibrators are streamlined, neutrally buoyant, and have control systems for changing depth, course and speed. Although multiple units of such devices could be simultanesously towed to increase the amount of acoustic power generated, simultaneous deployment of multiple acoustic sources complicates deployment, synchronization, and operation.
In addition to the high frequency, low power electromechanical transducers described above for submarine operations, slotted transducers have been used in loud speakers and in underwater sonar applications. Proposed uses for such transducers include pile drivers, trench diggers, gravel packers, replaceable knives or drills or surgical blades, sonic tools in oil wells, and sonobuoy and sonar installations.
Various forms of electromechanical transducers have been developed. U.S. Pat. No. 4,220,887 to Kompanek (1980) described a slotted electromechanical transducer having a resilient member in the slot for prestressing the transducer and for preventing contact between adjacent transducer ends. U.S. Pat. No. 4,651,044 to Kompanek (1987) specifically attempted to produce large amounts of power at low frequency, however the frequency range was in the order of "several kilocycles". U.S. Pat. No. 5,122,992 to Kompanek (1992) disclosed a transducer member having a closure member extending in a U-shaped configuration. The length of the closure member defined the bandwidth of the vibration frequency produced. In U.S. Pat. No. 5,267,223 to Flanagan et al. (1993), a compliant cover was bonded to a transducer shell.
Conventional seismic source technology does not efficiently provide high power, low frequency source energy from a simple operating system. There is, accordingly, a need for an improved seismic source generator for use in marine seismic operations. The seismic source generator should produce high power output up to and exceeding 190 dB re micro Pascal at one meter and should operate in a frequency range between 5 and 120 Hz. The seismic source generator should be compact to facilitate deployment and retrieval.